Supernumerary Robotic Limbs (SRLs) give you more limbs
than you'd normally have. In other words, they're not robotic
limbs designed to replace biological limbs that you might be
missing, but rather robotic limbs designed to augment the
number of limbs that you already have.

MIT researchers have been developing SRLs that can help
you do stuff that would be annoying, uncomfortable, or
impossible to do on your own. Their latest SRL prototypes
include one model featuring a pair of limbs that spring from the
shoulders and another with limbs that extend from the waist.

MIT's shoulder-mounted SRL is designed to assist in tasks
that take place over your head or in situations where your
other two arms are busy and you need a hand (literally) with
something. One example would be in a construction context,
where anything that needs to be attached to a ceiling has to be
held up and hammered or screwed into place at the same time.
Another example is trying to open a door when you're holding
something with both hands.

One SRL robot uses two arms mounted on the shoulders
such that the reaction forces on them are aligned with the
spine. Each arm has five degrees of freedom with
interchangeable and customizable end effectors, and the
complete systems weighs about 4. 5 kilograms ( 10 pounds).

What's tricky about having a pair of shoulder arms is
getting them to do what they're supposed to do without having
to control them with your other arms — which would kind of
defeat the purpose of the entire setup. Instead, the SRL
watches what you're doing with your arms to decide how it
should move. It does that by monitoring two inertial
measurement units (IMUs) that the user wears on the wrists. A
third IMU sits at the base of the robot’s shoulder mount to track the overall orientation and motion of the SRL.

The SRL uses the gyro and accelerometer data from the IMUs to make a prediction (based on a model that's
been created by demonstration learning) about what would be the most helpful proactive position for its own
arms. If you put your arms up above your head, for example, the SRLs raise above your head too because it figures
you're trying to hold something up. Using their SRL prototype, the researchers are testing different "behavioral
modes" to program the limbs to do what is desired.